Ghostrunning

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"Ghostrunning" (a.k.a. "Ghosting", "Ghost Mining") is the practice of deliberately allowing a Ghost to enter a level and convert normal gems into valuable diamonds. This also allows you to grab all other treasure in the level without being constricted by the time limit. Because Ghosts kill the Spelunker in one hit and can freely pass through walls, ghostrunning is an extremely dangerous technique, but it can result in amazingly high scores.

The technique is threefold:

Don't pick up any gems until the Ghost enters the level;

When the Ghost appears, lead it over the gems;

Don't allow yourself to get cornered.

Because ghostrunning is practiced in order to maximize scores, ghostrunners will also want to visit the City of Gold. In Spelunky Classic, the Hedjet negates the Ghost, so which level The Moai spawns in can significantly affect the total score.

To ghostrun, you need the following:

A way to dig treasure out of the walls.

The Tunnel Man is the easiest way, though sticky bombs still allow him to collect some extra treasure.

Without the Tunnel Man, you need a large amount of Bombs. High favor helps at this.

A way to quickly ascend the level to run laps around it, in order to avoid the Ghost and reach all treasure.

A shaft at the edge of the level made using 3 bombs and 3 ropes.

Ropes are the simplest way. As little as 3 ropes may be enough, if they are placed on or near the same edge of the level, either in a dug shaft or through a combination of natural openings and bombed holes.

The Climbing Gloves can be a better alternative, depending on your ability to manipulate the terrain. It's most effective to dig a clear vertical shaft in one or both edges of the level. This allows you to easily ascend the side wall with the Gloves, but if you're relying on bombs it's a costly technique.

The Jetpack is far superior to everything else, allowing free movement across the level without need to construct any shafts.

It is technically possible to "ghostrun" a level without either of the former, as you can still convert any visible gems to diamonds, and you can dig out more gold than you could if running out of time was an issue. This is not worth spending bombs and ropes, though; only do it with a Tunnel Man armed with Climbing Gloves or a Jetpack. In any case, this is only an issue in levels before the Udjat has spawned.

Ways to eliminate threats in the level.

A Shotgun is the best bet for eliminating enemies. Shopkeepers are priority 1.

Traps can be bombed, or dug away by the Tunnel Man.

Practice in manipulating the Ghost's path

Luck

In summary, the Tunnel Man's infinite digging ability significantly eases the requirements in multiple ways. Without him, you will need very specific equipment, or a large amount of basic equipment. The Tunnel Man will still greatly benefit from top-quality equipment. The less equipment or abilities you have, the riskier ghostrunning becomes.

DeLGato232, the originator of the term "ghostrun," created a thread at the official Spelunky forums which illustrates the method. It can be read here: